Gonzalo Zegarra
(CNN) — The deep landslides beneath the million-dollar homes of Rancho Palos Verdes were advancing at an almost imperceptible pace — until that changed.
This affluent Southern California coastal city, about 50 km south of Los Angeles, has long attracted people for its Pacific Ocean views and lush vegetation. But it sits atop a complex of slow-moving landslides that have been active since the 1950s, causing the land to shift a few metres a year. Recently, after heavy winter rains, the pace and magnitude of the movement have increased.
Last weekend, a drastic acceleration had devastating consequences.
The planet experiences the hottest summer ever recorded
Houses now sprawl unevenly across the distorted terrain, roads have caved in and more than 200 homes have been left without power. On Tuesday, Gov. Gavin Newsom declared a state of emergency in the city.
The sight of luxury waterfront homes teetering precariously on cliffs or partially swept away by the earth is not unfamiliar in this part of the United States. Landslides destroy homes, claim lives and raise fears for the future of communities.
But scientists warn that they will become more frequent as the climate crisis brings heavier rainfall and more powerful storms, reshaping the landscape.
Landslides depend on three factors: slope, rock type and climate, explains Alexander Handwerger, a landslide scientist at NASA’s Jet Propulsion Laboratory.
Rancho Palos Verdes sits on a bed of volcanic ash, deposited between 10 and 15 million years ago, that descends to the Pacific coast.
“It’s become a kind of clay mineral that can expand and become slippery when it gets wet,” said Gary Griggs, professor of Earth and planetary sciences at the University of California, Santa Cruz.
Various factors can trigger land movements, such as earthquakes and human activities. But rainfall is one of the most common.
When it rains, water seeps into the soil and penetrates into the lower layers. There, it can reduce the suction and friction that hold grains of soil or rock together, causing the soil to weaken and shift.
How food can help save the planet
Slopes always try to reach a stable angle, which depends on the climate they’re in, explains Dave Petley, an Earth scientist at the University of Hull in England. If the climate changes and rainfall becomes more intense, the slope “may now be too steep to be stable, so it will undergo a landslide or a series of landslides to find a new stable angle,” he told CNN.
In California, climate change is forcing the landscape to respond.
Over the past two years, atmospheric rivers — long columns of water in the sky from the tropics — have lashed the state with rain.
In February, an atmospheric river dumped record amounts of rain on Southern California, triggering hundreds of landslides and leaving at least nine people dead.
Rain ate away at the cliffs; a stark image shows a small cluster of mansions in Dana Point dangerously close to falling onto the debris-strewn beach.
Luxury homes are in danger of falling due to a landslide following heavy rains, in Dana Point, California, on Feb. 15, 2024. (Ted Soqui/Sipa USA/AP)
Scientists have discovered a clear link between the climate crisis and more intense rainfall. A warmer atmosphere can hold more moisture, resulting in more intense rain or snow when it falls, and warmer oceans fuel more powerful storms.
In California, climate projections suggest the state will experience less frequent but more intense rainfall in the future, especially from atmospheric rivers, which are expected to become more potent as the world warms.
The risk of landslides is clear, says Handwerger, who published a study on the topic in 2022. “We have looked across the state and we see that in wetter than average years, landslides accelerate.”
The climate crisis also poses other landslide risks. Rising sea levels and storm surges are eating away at cliffs. Hotter, drier summers are increasing the frequency and severity of wildfires, leaving the landscape vulnerable to landslides, Griggs said.
The 2018 mudslides that swept through Montecito, killing 23 people, came after the Thomas Fire, at the time the largest wildfire in California history, which incinerated trees and plants.
A firefighter stands on the roof of a house submerged in mud and rocks following a landslide in Montecito, California, in 2018. (Marcio Jose Sanchez/AP)
Landslides are, of course, a global phenomenon, and scientists are identifying climate change-driven landslide risks around the world.
Cyclone Gabriel in New Zealand triggered more than 140,000 mapped landslides, and possibly more than 800,000 in total, researchers say.
In July, a landslide triggered by heavy monsoon rains in the southern Indian state of Kerala killed at least 150 people. According to a scientific analysis, the rains were at least 10% heavier due to the climate crisis.
Atlantic currents could collapse in the 2030s
Climate change is not the only factor increasing the likelihood of landslides; human behaviour also plays a role.
Cutting into hillsides to make way for houses or roads can weaken them and the mountainsides themselves, making them unstable, explains Ugur Öztürk, a landslide scientist at the University of Potsdam and the German Research Centre for Geosciences GFZ.
The same can happen if too much water is added to the soil, Griggs said. In Southern California, “people wanted to pretend they lived in the tropics,” he said, “and they planted a lot of gardens that required a lot of watering.”
Deforestation is another factor. Tree and plant roots hold soil together, and ripping them out can destabilise it, said Petley of the University of Hull.
But, he added, “climate change is key.”
For those living in Rancho Palos Verdes, where the ground is now moving at up to 30 cm a week, the future of their community hangs in the balance. It is unclear when the movement will slow down, or whether they will be able to save their homes from being destroyed.
